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Lm317 to a 48v ac powersupply, or how to lower the voltage 10 v. Answered

So i have this transformer that puts out 48v ac.
I want to make a variable power supply, and it should put out 7 or more amps.
I could use lm317 with a bypassing transistor, but lm317's maximum input-output voltage differential is 40v,  so what should i do?
If anyone has ideas how to lower the transformer's voltage 10 volts that would be great.



Best Answer 5 years ago

The voltage can be lowered eleven ways that I can think of ;
  1. A New Transformer
  2. Bucking
  3. Capacitive
  4. Inducting
  6. Multistage
  7. Resisting
  8. SCR
  9. TRIAC
  10. Unwinding
  11. Zener

Ten is the best way after one way, if you can access the secondary
of your  xfmr and unwind some turns.
You do have a multimeter.
Also if the xfmr has a terciary low voltage winding then use the
second bucking way.

Mpilch is correct that you will need additional circuitry to get a 7Amp
output provided the xfmr is able to supply 336 VA or more.
Otherwise you will be getting it too Hot.


Unwinding is the most easy solution, but since i am going to order parts, i am going to order lm317hv-t it supports up to 48 volts, and some tip3055 transistors for the bypassing, but thanks for your answer.
The transformer is so heavily coated in epoxy, so unwinding could be difficult...

Won't work. 48V AC is likely to wind up more like 68V on the reservoir caps.

Hi , follow your Q, i think this.:

i imagine you have this ...48 VAC...(you tell us need use one LM317 and delivery 48vdc with 7Ampers
( i dont think so - because ic lm317 handle max 40VDC input)
but the most important one 317 give to you 1 amp, and to get mor ampered i think, you need put more LM317 in parallel, but please, like say mpilchfamily, read or understanding the fundamentals of all this, or check all data sheets ... and remember ---Take all the recommendations posible because you are playing with voltage and you must to know when is VCA and When is VCD , i mean in what point of your circuit you have thats voltages because could be dangerous for you and for around to you.

ending ... i think is possible have 7 A but at 40 VDC no 48 VDC....

No 317 can give you 7A output. You need an extra shunt transistor. And it will BURN so put it on a good heatsink !!!

I will loan myself "I could use lm317 with a bypassing transistor" .

First thing that needs to be done is rectify the 48V AC so you have DC to go into your regulator. Then you'll need to reduce the voltage to a level the LM317 can handle or use a different regulator. If you check the data sheet for the regulator it will tell you how ti dial in the voltage with that regulator. But to get 7A out of it you'll need to add some more circuitry.

Making a good power supply gets pretty complex. If you have trouble understanding how to get the voltages you want from the regulator then you will have a heck of a time trying to figure out the rest. When it comes to power supplies it's best to find an off the shelf solution rather then designing your own.